tg3: Create phy_flags and migrate phy_is_low_power

This patch deletes the link_config.phy_is_low_power flag and creates a
new phy_flags device member to store all phy related settings.  All the
code is converted accordingly.

Reviewed-by: Benjamin Li <benli@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>